The production fluid from an oil well includes varying proportions of oil, water and gas in which are entrained solid particles, hereinafter referred to as "sand". This mixture is normally fed to a phase separator in which settling under gravity occurs into an upper gaseous layer, a middle oil layer, and an lower water layer. These are removed through separate outlets from the separator. The sand naturally settles out in the bottom of the water layer in the tank and it will be undesirable to allow this to be discharged through the water outlet, not least because the sand particles will be coated with oil and it is unacceptable to discharge such coated particles with the water back into the environment. Consequently the coated sand particles are recovered from the bottom of the phase separator and, according to one method, supplied to a vessel fitted with impellers which rotate in opposite directions to suspend the contaminated particles in water, causing dynamic contact of the particles, which mechanically strips away the oil coatings from the solid particles. The clean solid particles may then be discharged to the environment but the carrier water has to be treated, normally by flotation, to remove the oil. This is difficult, costly, and takes space which is at a premium on, for example, a marine platform.
Other situations obtain where it is necessary to clean oil-coated sand particles, for example when an oil spillage contaminates a beach.